The invention relates to an X-ray microscope which includes a device for generating X-rays, which device is provided with:
means for producing a fluid jet,
means for forming a focused radiation beam whose focus is situated on the fluid jet.
A device for generating soft X-rays is known from the published patent application WO 97/40650 (PCT/SE 97/00697). The means for producing a fluid jet in the known device are formed by a nozzle wherefrom a fluid such as water is ejected under a high pressure. The means for producing a focused radiation beam are formed by a combination of a pulsating laser and a focusing lens which focuses the pulsating radiation beam produced by the laser in such a manner that the focus is situated on the fluid jet. Because of the high power density of the laser pulses, the laser light thus induces a plasma in the fluid jet, thus generating said soft X-rays. The cited patent application describes how these X-rays, notably those of a wavelength of 2.3–4.4 nm, can be used for X-ray microscopy.
Generating X-rays by way of pulsed laser plasma emission has a number of drawbacks.
A first drawback in this respect is due to the fact that it is necessary to operate the laser in the pulsating mode in order to achieve an adequate power density of the laser. The cited patent application mentions a power density of from 1013–1015 W/cm2 ; if this power is to be generated by means of a laser in continuous operation, an extremely large laser would be required. As a result, this known X-ray source produces only X-rays of a pulsating nature.
A further drawback of laser-induced plasma emission consists in the phenomenon that many particles (molecules, radicals, atoms (ionized or not), which usually have a high kinetic energy and may be very reactive chemically are present in the vicinity of the location where the X-rays are formed (the X-ray spot). The formation of these particles can be explained as follows: when energy is applied to the target (so the fluid jet) by means of laser light, as the intensity increases first the electrons of the outer shell of the target material will be ionized whereas the electrons of the inner shells, producing the X-rays, are excited only after that. The particles then formed could damage the sample to be examined by means of the X-ray microscope. In order to mitigate or prevent such damage, it is feasible to arrange an optical intermediate element (for example, a condenser lens in the form of a Fresnel zone plate) between the physical X-ray spot and the actually desired location of the X-ray spot, thus creating an adequate distance between the X-ray spot and the sample without seriously affecting the imaging properties of the X-ray microscope. Because condenser lenses are not very effective in the X-ray field, however, a considerable part of the X-ray power generated for the imaging in the X-ray microscope is thus lost. Moreover, some other types of condensers (for example, multilayer mirrors or grazing incidence mirrors) are very susceptible to damage by said high energetic particles.